Tesla's new home battery: energy revolution or pure hype?

Powerwall aims to fill a missing link to integrate wind and solar into the grid.


The hyperbolic clichés started flying mere hours after the May 1 announcement by Elon Musk, the eccentric CEO of Tesla Motors, that his company would soon begin mass production of the Powerwall, a battery for powering the home. Musk said the battery marked a “fundamental transformation of how energy is delivered across the earth.” Others called it the Holy Grail and a silver bullet against the vampire-esque utilities who are trying to bleed rooftop solar customers dry with added “grid charges.”

There’s no doubt that the Powerwall is a big deal, and that the utility-scale Powerpack is even more significant. Affordable, scalable, practical energy storage has long been considered the missing link needed to integrate significant amounts of wind and solar into the power grid. The Powerwall appears to be a big step in that direction. But how big of a deal is it, really? Is it, say, Segway big (a kind of neat thing for people with too much money to put in their garage)? Or is it cell phone big (a product that turns us all into tiny-screen-gazing humanoids with mutated thumbs)?

In order to help us figure it out, I've put on my energy-geek goggles to run through some of the main features of this potentially hip new gadget:

What is it? The Powerwall is a lithium-ion battery, not unlike the one in your laptop except that it’s four feet tall, three feet wide, seven inches deep, costs $3,000 and looks a bit like a giant MacBook that you mount on your wall. The Powerwall can be recharged with power generated from photovoltaic panels, drawing from “that handy fusion reactor in the sky known as the sun,” as Musk says. Then, just as your laptop battery can keep your cute kitten YouTube videos going for a couple of hours at the coffee shop where they’ve banned you from using the outlets, the Powerwall can keep the whole home juiced for a few hours after the sun’s gone down. The daily-cycling Powerwall has a 7 kilowatt-hour capacity, 70 to 100 times that of a laptop battery. A slow-cycling 10 kWh version is intended to be used as backup in the event of a prolonged blackout. Up to nine Powerwalls can be “stacked” in one home, combining the capacity.

What’s so revolutionary about a big battery? There are already plenty of home batteries on the market that function similarly to the Powerwall, charging while the sun shines and supplying electricity when it doesn’t. Off-grid homes typically use a battery bank consisting of rows of deep-cycle lead-acid batteries such as those used to power golf carts or for boats. “The issue with existing batteries is that they suck,” says Musk “They’re really horrible.” They are clunky, ugly, they require maintenance and don’t work well in the cold and the heat and they need dedicated, well-ventilated battery “rooms." They’re also expensive. The Powerwall is sleek-looking, compact, doesn’t need its own room and is easily mounted to a wall, indoors or out. And it’s made specifically for homes, not boats or golf carts. The Powerwall is more affordable than any other lithium-ion option, but appears to be more expensive on a $/kWh basis than many lead-acid options, for now. Expect the price to drop once the Tesla "gigafactory" in Reno is up and running and producing these on a mass scale.

Solar panels at Second Mesa, Hopi. The Powerwall will make it easier and cheaper to install off-grid solar systems.
Jonathan Thompson

Who’s going to buy a $3,000 battery? I suspect the first wave of orders for these things will come from folks who are off-grid, people who already have invested in the solar array, the DC-AC inverter, and other accessories needed to run one’s lights, appliances and computers off of photovoltaic power, and who are ready to dispense of that nasty battery room and everything in it. Next up will be folks who are building new homes beyond the reach of the grid and can’t or don’t want to pay to extend the power lines. And hundreds of homes on the Navajo Nation, to name one example, lack electricity. The Powerwall and similar technology (Musk open sources his patents to allow others to build upon his designs) should make it a bit easier and affordable for individuals, governments or NGOs to equip those homes with off-grid solar systems. And then there are those who have grid-connected solar arrays that have a generating capacity that meets or exceeds their total electricity consumption and who want to go off-grid — mainly on principle — because they want to stick it to the utility that has been tacking extra “grid usage” fees onto their electric bill. The Powerwall may be the incentive they've been waiting for to make the leap.

That's not a lot of people, though. Fact is, modern American homes and the appliances and devices that fill them up are serious electricity guzzlers, requiring more power than a practically-sized solar array and battery bank can provide. A cheaper, sleeker battery’s not going to free the masses from the grid; first, we need to change our consumption habits while also rethinking the way we design and build our homes and appliances to make them more efficient. Then comes the battery.

So, only off-grid hippie survivalists are going to buy this thing? Actually, even folks without any solar panels can benefit from the Powerwall, if they can afford it. During a short blackout, a Powerwall or three could keep the lights on. If you’re on a time-of-use rate plan with your electric company, then you could save a bit of money by charging the battery with the grid when rates are low (typically the middle of the day, when solar generation is peaking and consumption is still low), and powering your house with it when rates are high (in the late afternoon/early evening when AC is cranking up and folks are getting home from work). It would take years to offset the $3,000 price tag, though. Fact is, the purely economic case for buying a Powerwall is pretty weak in just about any situation. But then, the economic case for buying, say, a giant flat-screen television or Apple's new watch is far more rotten.

Is it a weapon against solar-hating utilities? While a number of utilities — in Arizona, for example — are taking shots at customers with rooftop solar by adding extra fees and the like, many places still have decent net-metering programs. The best are those which allow every excess kWh that a customer’s solar panels put into the grid to offset one taken from the grid, regardless of the time of day. In that case, the grid serves as a sort of battery for the customer’s solar array: He “charges” it during the daytime, and draws from it at night. The Powerwall would provide little benefit to the customer aside from blackout backup. But a customer under a less favorable net-metering regime could use the Powerwall to his advantage, charging it with solar when electricity rates are low and discharging it when they’re high and solar’s not cranking (see time-of-use discussion above).

The biggest benefit of the Powerwall, though, will come if it’s deployed en masse. Grids with lots of solar, either utility-scale or rooftop, must grapple with the dreaded “duck graph,” a term describing the shape of net power demand (demand minus solar generation) over the course of the day. The curve goes into a trough in mid-day, when solar is cranking but demand is low, but then spikes in late afternoon as solar generation plummets at the same time that air conditioners kick in and folks get home from work and flip on their televisions. Grid operators must fire up expensive, polluting natural gas plants to “follow” the increased demand. A giant battery could replace the backup power plants by leveling out both trough and spike in the duck graph.

Musk says Tesla is also developing the Powerpack, a utility-scale battery, cheaper than other available options, designed for “infinite scalability,” to help with that. But the same effect could be achieved by combining the storage power of thousands of Powerwalls scattered among customers’ homes — distributed storage backing up distributed generation. A utility driven by common sense and concern for the environment would subsidize customers’ Powerwall and solar system purchases for this very reason. But utilities are driven by profit, not common sense, so don't hold your breath.

Jonathan Thompson is a senior editor at High Country News. Homepage photo courtesy Solar City, which will be offering the Powerwall as part of its home solar systems.